1. Field of the Invention
The present invention relates to a method of preventing drill string overflow, and more particularly, but not by way of limitation, to an improved method of lowering the level of drilling fluid in a drill string below the point at which drill pipe sections are added to or removed from the drill string.
2. Description of the Prior Art
In the drilling of well bores penetrating subterranean formations, it is frequently necessary to withdraw the drill string from the well bore to change the drill bit, replace worn out drill pipe sections, etc. In addition, during drilling it is necessary to add drill pipe sections to the drill string as the well bore is deepened.
During drilling, drilling fluid is circulated down the drill string, through ports or nozzles in the drill bit attached to the lower end thereof and up the annulus between the drill string and the walls of the well bore to the surface. When drilling is stopped for the addition or removal of pipe sections to or from the drill string, because the drilling fluid in the annulus contains cuttings from the formation being drilled, it has a higher density that the drilling fluid within the drill string. This difference in drilling fluid density causes the level of the drilling fluid in the drill string to be higher than the level of drilling fluid in the annulus, often higher than the level at which drill pipe sections are connected to or disconnected from the drill string, i.e., the level of the drilling rig floor. In addition, the nozzles in the drill bit at the bottom of the drill string restrict the drainage of the drill string as it is raised. The combination of the above factors results in the level of drilling fluid in the drill string and drilling rig conduits attached thereto usually always being above the rig floor which in turn causes drilling fluid to overflow at the point of drill string disconnection. The overflow of drilling fluid results in the loss thereof, and more importantly, hinders and causes unsafe conditions for personnel.
Methods have heretofore been developed and used for expediting the draining and equalization of drilling fluids to thereby prevent drilling fluid overflow when adding or removing drill pipe sections to and from drill strings. U.S. Pat. No. 2,595,715 issued May 6, 1952 to Sloan discloses a method for clearing the kelly of drilling mud each time it is hoisted for the addition of a new drill pipe section. In accordance with such method, compressed air or other gaseous fluid at superatmospheric pressure is introduced into the kelly to force the drilling fluid downwardly through the drill bit followed by the releasing of the air pressure. While this method has achieved success, it is time-consuming in that a large quantity of air or gas must be introduced into the drill string in order to lower the level of drilling fluid in the drill string whereby drilling fluid overflow does not occur. Consequently, the method has not received wide acceptance and drilling rig operators have generally heretofore accepted drilling fluid spillage as a necessary part of the operation of adding or removing drill pipe sections to or from drill strings.
An improved method of preventing drilling fluid overflow and formation fluid blowouts is described in my U.S. Pat. No. 3,963,077 issued June 15, 1976. In such method, a gas is introduced into the annulus between the well bore and the drill string until a predetermined gas pressure is reached therewithin whereby drilling fluid is displaced downwardly within the annulus and upwardly within the drill string. The gas pressure is then released from the annulus and the fluid levels in the drill string and annulus are allowed to equalize whereby the level of drilling fluid in the drill string is below the level at which drill pipe sections are disconnected therefrom or connected thereto. This method has also not received wide acceptance in that large quantities of gas are required for displacing the drilling fluid and the equalization of the drilling fluid levels in the drill string and annulus still require the drilling fluid to flow through the restricted nozzles in the drill bit making the method time-consuming.
By the present invention an improved method of preventing drilling fluid overflow when adding or removing drill pipe sections to or from a drill string disposed in a well bore is provided which is simple to carry out, which does not require large quantities of gas and which, consequently, is less time-consuming to carry out.